In many electronic circuit systems, the system is made up of many modules. Each module is a printed circuit card that carries many circuit components, including integrated circuits. Each module is a printed circuit card of a standard size large enough to yield efficient, economical manufacture. Each printed circuit card or module is mounted in one of a plurality of slots in a metal frame or rack.
The frame or rack is typically constructed with a fixed, finite spacing between printed circuit card slots, with a spacing determined by the height of the electronic components that are expected to be mounted on the overwhelming majority of the circuit cards or modules. Complex electronic systems can be very large and thus occupy a lot of space. Therefore, generous spacing between card or module slots on a frame or rack is to be avoided.
Each printed circuit card or module has some form of connection facility or connector on at least one of its four edges. This connection facility is plugged into a mating connection facility or connector on a printed wiring backplane or motherboard of the system. The backplane carries power busses to all of the printed circuit board connectors, as well as signalling, data, and timing busses. All of this serves to meld the several printed circuit cards or modules into a formation that constitutes the desired system.
Each printed circuit card or module is specifically manufactured to perform one or more very specific functions within the system. In order to maximize the use of printed circuit card "real estate," the circuitry to perform two or more separate functions can and often is mounted on the same printed circuit card or module. However, when a module is designed and manufactured to contain the circuitry for more than one function, there may be systems in which one but not all of the functional circuits on the module are needed. This can be wasteful.
Also, if a fault is isolated to one of the circuit functions of a multi-function card or module, the entire module card is usually replaced and discarded or sent for factory repair, not just the faulty function circuit. Therefore, the attempt to achieve manufacturing economies can actually defeat maintenance and repair economies.
Attempts have been made to mount more than one smaller, functional printed circuit cards (sub-modules) on a single standard-size module card. Added connectors and clips to hold the sub-module cards to the module card usually occupy an inordinate amount of that valuable "real estate" on the module card. Still more "real estate" is lost in order to accommodate the space needed to slide the sub-modules into their connectors.
Also, since both the original module card and the sub-module card are now sandwiched together in one slot spacing in the frame or rack, there is either a reduction in the height permitted for the circuit components that can be mounted on the sub-module, or an adjacent slot in the frame must be left vacant in order to accommodate the now thicker sandwich module card. Neither of these alternatives is desirable.
In many electronic systems, connection must be made both to the edge of the module card that connects to the backplane board and to the opposite edge of the module card. Such opposite-edge connection needs are common in the field of electronic testing systems. In the testing field, it is often desired to connect various test instruments or units under test to the opposite edge of the module card. When placing more than one circuit function on a module card, it is necessary that provision be made to provide access from the opposite edge of the module card to each of the different functional circuits on the module card.
When a module card is built to accommodate sub-module cards, provision must be made for interconnection of the sub-module cards to both edges of the module card. This is even more important when the module card is intended to be of a general-purpose nature, with the ability to accept a number of different types and sizes of sub-module card at any one of several positions on the module card.
When a general-purpose module card is constructed, it is not always possible to plan exactly where each and every backplane conductor will connect to a sub-module. Therefore, it is sometimes difficult to plan conductor routings from the backplane, through the general-purpose module, and to the sub-module.